Wood delignification chemical process using ammonium magnesium bisulphite as the active reagent

ABSTRACT

The present innovation consists of the use of a reagent solution (FINAL COOKING ACID) where ammonium magnesium bisulphite is the active chemical. This new process is particularly useful to those industrial plants where the cellulose pulp is obtained by the magnesium acid process. The present innovation consists of the introduction of ammonium ions in the cooking acid used in such plants (initial cooking acid). The final cooking acid is obtained by the reaction of the initial cooking acid with ammonium hydroxide. Experimental results show the superiority of the present innovation over the magnesium acid bisulphite pulping process which is expressed by production of cellulosic fibres with higher mechanico-physical index values, decrease in the percentage of uncooked material, increase in wood yield and less environmental impact.

[0001] Several sulphite wood delignification process with industrialapplication have been known for a number of years. The distinctionbetween them is based on the active chemical present in the reagentsolution (cooking acid).

[0002] The compounds used as active chemicals are all monocationicbisulphites, mainly of calcium, magnesium, sodium or ammonium. Thepulping process with calcium bisulphite is an exception, when dolomiteis used to prepare the acid.

[0003] In the last 30 years the bisulphite cellulosic pulps have beenlosing competivness to KRAFT pulps, which are obtained by a chemicalprocess in an alkaline medium.

[0004] The major disadvantage of bisulphite pulps is the weakermechano-physical characteristics of their cellulosic fibres whencompared with those obtained by the KRAFT process.

[0005] SU-A-1359387 discloses a pulping process for wood from deciduoustrees. This process is a semi-chemical process, the liquor pH isalkaline (8-9.5), the liquor composition is MgSO₃+(NH₄)₂SO₃+[NH₄⁺=f(pH)] and the active chemical agente is ammonium magnesium sulphite.U.S. Pat. No. 463,499 discloses a special hardwood sulphitedelignification process using exclusively the ammonium base. GB-A-734216discloses a process for the treatment of wood chips with sulphur dioxideprior with digestion with alkali, namely caustic soda. U.S. Pat. No.4,141,787 discloses a special acid bisulphite process using calcium ormagnesium base, followed by the feed of liquid So₂. DD-151581 disclosesa special wood delignification process using a monocationic bisulphite.

[0006] The present innovation refers to a chemical process for wooddelignification and consists of the use of a reagent solution whereammonium and magnesium bisulphites are together the active chemicals.

[0007] The combination of both ammonium and magnesium cations asbisulphite is the novelty of this application.

[0008] This new pulping process is particularly useful to thoseindustrial plants where cellulose pulp is obtained by the magnesium acidbisulphite process.

[0009] The expression “ammonium magnesium bisulphite” means an aqueousunsaturated solution containing an ionic mixture of ammonium andmagnesium bisulphite salts, fully dissociated: HSO₃ ⁻+Mg²⁺+NH₄ ⁺, apH=2-4 kept through an excess of dissolved sulphur dioxide.

[0010] The bisulphite cooking is an aqueous solution of bisulphitecontaining an excess of SO₂ called “True free”.

[0011] The composition of bisulphite cooking acid is determined by thetotal amount of dioxide sulphur in liquor and by ratio of Combined SO₂to Free SO₂. The amount of Free SO₂ is the total of the True Free plusone half of SO₂ in bisulphite form. It is the amount of SO₂ in excess ofthe amount theoretically required to form monosulphite. By the sametoken, the Combined SO₂ is equal to one-half of SO₂ in the bisulphite,or equal to the amount of SO₂ held as monosulphite.

[0012] The specific cooking acid of magnesium acid bisulphite pulpingprocess is, physically, a gas-liquid system. The liquid phase is amagnesium bisulphite and sulphur dioxide aqueous solution in equilibriumwith the SO₂ gas phase. The system is kept at temperature and pressurevalues of about 50° C. and 2 Kgcm⁻², respectively.

[0013] This particular cooking acid will be referred further on thisdocument as the INICIAL COOKING ACID.

[0014] The basis of the present innovation is the inclusion of theammonium ion in the initial cooking acid, as described by the followingequation: INITIAL COOKING ACID: Mg(HSO₃)₂ (aq) + SO₂(g) + Ammoniumhydroxide addition: NH₄OH(aq) = Reaction product: NH₄(HSO₃)(aq)

[0015] The cooking acid obtained by this reaction will be referred to inthis document as the FINAL COOKING ACID.

[0016] FINAL COOKING ACID composition:

Mg(HSO₃)₂+NH₄(HSO₃)+[SO₂=f(pH)]

[0017] The amount of SO₂ stoichiometrically in excess is a function ofpH.

[0018] The addition of ammonium hydroxide must drive the initial pHvalue of inicial cooking acid (1.5) to a final value between 2 and 4,depending on the needs of the process. This operation could be performedin a tank with the initial cooking acid under pressure (accumulator) orin the reactor (digester).

[0019] In the FINAL COOKING ACID the percentage of total SO2, free pluscombined forms, must be kept within 4% to 7%, depending on therequirements of the process, the percentage of combined SO₂ beingbetween 1.8% and 3.0%. Preferably, the total percentage of SO₂ totalshould be between 5% and 6%, the percentage of combined SO₂ beingbetween 2.2% and 2.7%.

[0020] The pulping process operated at a higher pH value, such as thatattained after ammonium hydroxide addition, decreases the wooddegradation process, enables the production of cellulosic fibres withhigher mechanico-physical index values, and decreases the specificconsumption of wood.

[0021] On the other hand, the presence of ammonium ions accelerate thereaction process decreasing both wood chip impregnation time andpercentage of uncooked material. This latter effect improves the woodyield of the process.

[0022] The experimental results obtained with hard wood, namelyeucalyptus, show the superiority of the present innovation over themagnesium acid bisulphite pulping process, which is expressed by.

[0023] a—Production of cellulosic fibres with a highermechanico-physical index values.

[0024] b—Decreased percentage of uncooked material.

[0025] c—Increased wood yield.

[0026] d—Less environmental impact.

1. Chemical process for wood delignification characterised by the use ofan aqueous unsaturated solution containing an ionic mixture of ammoniumand magnesium bisulphite salts (herein called ammonium magnesiumbisulphite reagent), fully dissociated: HSO₃ ⁻+Mg²⁺+NH₄ ⁺, a pH=2-4 keptthrough an excess of dissolved sulphur dioxide, where [SO2]=f(pH)], thepercentage of total SO₂, free plus combined forms, being kept within 4%to 7%, wherein the percentage of combined SO₂ is from 1.8% to 3.0%. 2.Chemical process for wood delignification, according to claim 1,characterised by a Final Cooking Acid containing: % SO₂ Total 5-6, % SO₂Combined 2.2-2.7 and a pH value between 2 and 4 at which the processmust be run by the controlled addition of NH₄OH to the initial cookingacid, whose pH is nearly 1.5.
 3. Chemical process for wooddelignification, according to claim 2, which being applied to eucalyptusand taking a monocationic acid bisulphite process, namely magnesium, asreference, is characterised by a pulp production having cellulosicfibres with higher physical properties, mainly tensile Index end TearIndex respectively 25% and 15%, at least higher
 4. Chemical process forwood delignification, according to claim 2, which being applied toeucalyptus and taking a monocationic acid bisulphite process, namelymagnesium, as reference, is characterised by a decrease in thepercentage of uncooked material nearly 30% and an increase in the woodyield nearly 4-5%.
 5. Chemical process for wood delignification,according to claim 2, which taking a monocationic acid bisulphiteprocess, namely magnesium, as reference, is characterised by thesignificant improvement of the environmental impact resulting from thelower amount of sulphur dioxide released to the air when the cook isdischarged.
 6. Process for preparing the ammonium magnesium bisulphitereagent referred to in claims 1 to 5, characterised by having asreaction reagents the initial cooking acid and ammonium hydroxide,according to the equation: INITIAL COOKING ACID: Mg(HSO₃)₂ (aq) +SO₂(g) + AMMONIUM HYDROXIDE NH₄OH(aq) = ADDITION: REACTION PRODUCT:NH₄(HSO₃)(aq)

FINAL COOKING ACID COMPOSITION: Mg(HSO₃)₂+NH₄(HSO₃)+SO₂(diss.),whereinthe two salts are fully dissociated in aqueous unsaturated solutioncontaining at pH=2-4 kept through an excess of dissolved sulphurdioxide, where [SO₂=f(pH)], the percentage of total S0 ₂, free pluscombined forms, being kept within 4% to 7%, wherein the percentage ofcombined SO₂ is from 1.8% to 3.0%.
 7. Process according to claim 6,characterised by a Final Cooking Acid containing: % SO₂ Total 5-6, % SO₂Combined 2.2-2.7 and a pH value between 2 and 4 at which the processmust be run by the controlled addition of NH₄OH to the initial cookingacid, whose pH is nearly 1.5.″